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Alkaline-earth phosphates efficient in the dehydration of lactic acid to acrylic acid were previously shown to contain a surface mono/dihydrogen phosphate amorphous layer composed of M2+cations and both P═O and POH groups. In this work, acidic properties of such a layer were determined combining Fourier transform infrared (FTIR) spectra achieved at the dehydrated state and under water vapor and density functional theory (DFT) simulations of nondefective and defective MPOH structure. The FTIR spectra of adsorbed pyridine and lutidine revealed the presence of moderate Lewis acid sites (LAS) and of POH groups interacting by H-bonding without significant protonation. DFT calculations were key to interpret FTIR spectra after adsorption of NH3: when solely adsorbed, NH3interacts with the LAS on both the nondefective surface and the defective surface, whereas the POH for which H points up toward the gas phase are reoriented downward. Brønsted acid sites (BAS) were shown to form under water vapor. This phenomenon was shown by DFT to arise from a more acidic character of HPO42–species for the nondefective surface and casual formation of nondefective surface leading to higher amount of H2PO4–species, which are more acidic BAS.

The present study is a part of an experimental microkinetic approach of the selective reduction of NOxto N2with NH3in excess of O2on V2O5/WO3/TiO2catalysts (NH3-selective catalytic reduction (NH3-SCR) reaction). Water is always present either in the reactive gas mixtures representative of industrial processes or produced by the reaction. This suggests that H2O may modify the coverage of the pivotal adsorbed NH3intermediate of the reaction by either a competitive adsorption or reactions (i.e., formation of NH4+). In the temperature range of interest for NH3-SCR (T≥ ≈423 K), Fourier transform infrared spectroscopy and volumetric measurement using a mass spectrometer are used to study the impacts of the NH3-H2O coadsorption on the coverages of adsorbed NH3(molecular adsorption) and H2O (molecular and dissociative adsorption) species on two sulfated solids: a 0.7% V2O5/9% WO3/TiO2NH3-SCR catalyst and its TiO2support. Regardless of the solid, it is shown that at the NH3-H2O coadsorption equilibrium, (a) NH3dominates the adsorption on the Lewis sites (i.e., the introduction of NH3at the H2O adsorption equilibrium displaces H2Oads-Lspecies at the benefit of NH3ads-Lspecies) and (b) the introduction of H2O at the NH3adsorption equilibrium increases significantly the amount of adsorbed NH4+species. This is ascribed to the H2O dissociation, which is operant on a small number of sites forming new Brønsted sites without a strong impact on the amount of Lewis sites. The surface composition of the solids has a limited impact on the coverages during the NH3-H2O coadsorption except on the fact that the NH4+species is more stable on the NH3-SCR catalyst. In Part 6 of the present study (10.1021/acs.jpcc.8b05847), it is shown that the present experimental data are consistent with the mathematical formalism of a competitive Temkin model (named Temkin-C) developed without major approximations. The experimental procedure (present study) and the mathematical Temkin-C formalism (Part 6) can be applied for all solids having a significant IR transmission, thus offering a method to study the surface acidity during realistic experimental conditions (in the presence of H2O), which is of interest for different catalytic processes such as NH3-SCR and alcohol dehydration.

The present study is a part of an experimental microkinetic approach (EMA) of the selective reduction of NOxto N2with NH3in excess O2on V2O5/WO3/TiO2catalysts (NH3-SCR process). In the temperature range of interest for NH3-SCR (T≥≈ 473 K) and for three TiO2-based solids (sulfated and sulfate-free TiO2supports and a sulfated 0.7% V2O5/9% WO3/TiO2catalyst), FTIR spectroscopy and volumetric measurements with a mass spectrometer are used to study the impacts of the NH3–H2O coadsorption on the coverages of (a) the molecularly adsorbed NH3species and (b) the molecularly and dissociated H2O species on Lewis and Brønsted sites. Whatever the solid, it is shown that NH3dominates the molecular coadsorption on the Lewis sites. However, this does not prevent the dissociative H2O chemisorption on a small amount of Lewis acidic sites, leading to an increase in the amount of OH groups. On the two sulfated solids, these OH groups increase the amount of adsorbed NH4+species as compared to the NH3adsorption equilibrium. For the sulfate-free TiO2solid having weak Brønsted sites, the switch between the NH3adsorption equilibrium to the NH3–H2O coadsorption equilibrium is associated to the production of a small amount of NH3due to the displacement of NH3ads-Lspecies by H2O dissociation (competitive adsorption). It is shown that these experimental data are consistent with an original development of a competitive Temkin model (named Temkin-C), taking into account the individual heats of adsorption of NH3and H2O species at different coverages in the absence of competition. The EMA and Temkin-C model developed in the present study can be applied to all solids having a significant IR transmission offering a method to study the surface acidity during realistic experimental conditions (in the presence of H2O), which is of interest for different catalytic processes such as NH3-SCR and alcohol dehydration.

The present study is a part of an experimental microkinetic approach of the removal of NOxfrom coal-fired power plants by reduction with NH3on V2O5/WO3/TiO2catalysts (NH3-selective catalytic reduction, NH3-SCR). It is dedicated to the characterization of the heats of adsorption of molecularly adsorbed H2Oadsspecies formed on sulfate-free and sulfated TiO2supports. Water, which is always present during the NH3-SCR, may be in competition and/or react (formation of NH4+) with the adsorbed NH3species controlling the coverage of the adsorbed intermediate species of the reaction. Mainly, an original experimental procedure named adsorption equilibrium infrared spectroscopy (AEIR) previously used for the adsorption of NH3species on the same solids is adapted for the adsorption of H2O. At Ta= 300 K and for PH2O≤ 1 kPa, three main H2Oadsspecies are formed (associated with a minor amount of dissociated H2O species) on the two TiO2solids. The species are identified by the positions of their IR bands in the 3750–3000 cm–1range. Considering the decreasing order of stability, they are (a) coordinated to strong (L2) and weak (L1) Lewis sites and denoted H2Oads-L2and H2Oads-L1, respectively, and (b) hydrogen bonded to the H2Oads-Lspecies and on O2–/OH sites of the solids (denoted H2Owads). The three species have a common well-defined δH2OIR band at a position in the range 1640–1610 cm–1according to the total coverage of the surface. According to the AEIR method, the evolution of the intensity of this IR band during the increase in the adsorption temperature Tain isobaric condition provides the evolution of the average coverage of the three species and then to their individual heats of adsorption as a function of their coverage. It is shown that there are no significant differences on the two TiO2solids. In particular, the heat of adsorption of the H2Oads-L2species varies from ∼114 to 61 kJ/mol at low and high coverages respectively, indicating that it can be present in the experimental conditions of the NH3-SCR. In a forthcoming article, the competitive chemisorptions and reaction between adsorbed H2O and NH3species are studied and modeled on the TiO2supports and model and commercial V2O5/WO3/TiO2catalysts.

The adsorption of x% CO/He (x= 1 and 2) on a reduced 10 wt % Co/Al2O3catalyst at an adsorption temperature of 560 K > Ta> 420 K leads to the formation of a linear CO species on Co° sites (denoted LCo°species) with an infrared (IR) band at ≈2030 cm–1at full coverage. This adsorption is associated with the formation of carbon that is implicated in the reconstruction of the surface of the cobalt particles, leading to the progressive transformation of the LCo°species into a new linear CO species on Co°C sites of the reconstructed surface with an IR band at ≈2060 cm–1. An experimental microkinetic approach of the reconstruction process via the LCo°→ LCo°Ctransformation reveals the impact of different kinetic parameters such as the reaction temperature, the CO partial pressure, and the presence of hydrogen. During the reconstruction the coverages of the LCo°and LCo°Cspecies remain very high (>0.95) due to their high heats of adsorption determined by the Adsorption Equilibrium Infra Red spectroscopy method. It is shown that the rate of the reconstruction process is controlled by that of the disappearance of the LCo°species via its dissociation into adsorbed elemental carbon and oxygen species according to a pseudo first kinetic order elementary step involving a small amount of free cobalt sites (≈4 × 1012sites/cm2of cobalt particles). The impact of the reaction temperature on the LCo°→ LCo°Ctransformation indicates that (a) the activation energy of the LCo°dissociation which controls the reconstruction process is ≈125 kJ/mol and (b) the formation of elemental carbon via the disproportionation of the LCo°species seems unlikely. These two conclusions are consistent with literature data on density functional theory (DFT) calculations. For adsorption temperature >560 K, the formation of superficial graphitic like species overlaps the reconstruction process. In the presence of hydrogen, the reconstruction is observed for H2/CO ratios <3 but not for higher values (i.e., H2/CO = 10).